Subterranean “sliding” drilling operation typically involves rotating a drill bit on a downhole motor at the remote end of a drill pipe string during a drilling operation. Drilling fluid forced through the drill pipe and downhole motor rotates the drill bit. The assembly is directed or “steered” from a vertical drill path in any number of directions, allowing the operator to guide the wellbore to desired underground locations. For example, to recover an underground hydrocarbon deposit, the operator may drill a vertical well to a point above the reservoir and then steer the wellbore to drill a deflected or “directional” well that penetrates the deposit. The well may pass horizontally through the deposit. Friction between the drill string and the wellbore generally increases as a function of the horizontal component of the wellbore, and slows drilling by reducing the force that pushes the bit into new formations.
Such directional drilling requires accurate orientation of a bent segment of the downhole motor that drives the bit. Rotating the drill string and adjusting the angular position of the pipe at the surface using a top drive or rotary table change the orientation of the bent segment called the toolface. To effectively steer the assembly, the operator must first determine the current toolface orientation, such as via measurement-while-drilling (MWD) apparatus. Thereafter, if the drilling direction needs adjustment, the operator must rotate the drill string to change the toolface orientation.
As various drilling parameters, such as weight on bit, flow rate, amount of bite the bit is making in the formation, rate of penetration, and/or other parameters change, the toolface position will be changing as well. It is desirable to accurately control the toolface position and keep the toolface in one position to allow for overall better steering control and a smoother wellbore. Traditionally, toolface feedback has been done manually by waiting for the MWD apparatus to communication (e.g., telemeter) the current toolface position to surface. The directional driller would then make any corresponding changes to the quill position to offset the error in the toolface as compared to the desired toolface. Such a control feedback loop is very slow and allows for a large amount of toolface position error while drilling, resulting in less than desirable steering control and wellbore consistency, ultimately resulting in longer “sliding” times and a reduced rate of penetration (ROP), and thus increasing the cost of the drilling process.
Automating toolface control while sliding can utilize a differential pressure (DP) measurement at the surface of the drilling rig to estimate toolface changes more quickly than can be telemetered by a downhole MWD tool. This allows for faster feedback to the control loop that can improve algorithms that control toolface orientation.
One problem with this method is that the relationship between DP and toolface changes depending on a variety of factors, such as depth and geometry of the drill string and formation, motor performance, bit performance, flow rate, and weight on bit. In order to estimate the expected change in toolface position relative to a change in differential pressure, an experienced directional driller is typically required to create a table to define this relationship, and the performance of the system is limited by the accuracy of the directional driller's estimation(s).
Thus, drilling a well typically involves a substantial amount of human decision making and/or analysis of relationships of various parameters during the drilling process. While a directional driller may have drilled other boreholes in the same region and so may have some similar experience, it is impossible for a human to mentally track all the possible inputs and factor those inputs into a decision. This can result in expensive mistakes, as errors in drilling can increase drilling costs and/or time.
Reorienting the toolface in a wellbore can be very complex, labor intensive, and often inaccurate, and thus, automated methods and systems are desired. This becomes increasingly true as wells are drilled faster and thus ROP levels increase due to greater added power.